sql runs fast in ssms slow in asp.net

I have been having this problem for a couple of weeks now. The problem is that the query takes 4-5 minutes to run on the website and at most 2 or 3 seconds to run in ssms. Also I found that after I make a change to this query like adding the customerId variable it will start running quickly on the web page but by the next day it is slow again. The query in question is this one:

DECLARE @customerID INT
SET @customerID = @CustID
DECLARE @MyTable table(
Iden int NOT NULL IDENTITY(1,1),
ProductID int)

INSERT INTO @MyTable(ProductID)
SELECT P.ProductID FROM Product P WITH (NOLOCK)
left join Compunix_ProductMMY cpmmy with (nolock) on p.ProductID = cpmmy.ProductID
left join Compunix_CustomerMMY ccmmy with (nolock) on ccmmy.mmyid = cpmmy.mmyid
WHERE P.Deleted=0 AND P.Published=1 and (ccmmy.customerid = @customerID OR cpmmy.productid IS NULL)

SELECT c.Name, c.SeName, c.CategoryID
FROM Category c WITH (NOLOCK) 
JOIN ProductCategory PC With (NOLOCK) ON C.CategoryID = PC.CategoryID
JOIN @MyTable MT ON PC.ProductID=MT.ProductID
WHERE C.Published = 1
GROUP BY c.Name, c.SeName, c.CategoryID
ORDER BY c.Name

I have the same query running on 2 other site which work just fine. The only difference between the sites is that they are run on different databases and the slow site has a bit more than double the products (54000 products) on it compared to the other 2. All three sites and their databases are hosted on the same machine.

Answer

Chances are that you are hitting on a problem with parameter sniffing.

I suggest reading Slow in the Application, Fast in SSMS? by Erland Sommarskog to get a full understanding of the issue (long article but very good).